Hybrid laser electric arc welding processes are widely used in industrial manufacturing. Hybrid laser electric arc welding is a combination of a laser welding process and an electric arc welding process, where a laser beam and an electric arc are directed to a single interaction zone at the work piece or work pieces that are to be welded. The laser beam may produce a high intensity amounting to around 1 MW/cm2. The laser beam will create a vapor cavity, known as a keyhole, having a high depth to width ratio. The arc produces generally a substantially lower intensity, around 1 kW/cm2. The focal spot of the arc is however substantially larger. As a result, the seam from an arc welding process is wider. For this reason, the bridging of gaps is considerably better when using an arc welding process. In a hybrid laser electric arc welding process the benefits of the two welding processes are combined, which result in a welding process with a higher welding speed and weld depth in comparison each welding process taken alone. The ratio of the power input from respective process will determine which welding process predominate the hybrid laser electric arc welding process. The hybrid laser electric arc welding process takes place under a shielding gas where the laser beam and the arc act on the work piece at essentially the same position.
An example of a hybrid laser electric arc welding process is disclosed in U.S. Pat. No. 7,288,737, which relates to a method for laser/MIG hybrid welding by depositing molten material into at least a portion of a bevel formed between edges to be welded together. It is suggested that the method disclosed in U.S. Pat. No. 7,288,737 is suitable for forming a weld seam between two edge portions of a work pieces having a thickness up till 60 mm.
It has however been shown that it is difficult forming a joint of high quality using prior art methods. Problems that may arise include a difficulty in achieving a stable welding process when forming weld seams between edge portions of a work piece or work pieces that are thick. In an unstable welding process the penetration into the work piece may vary such that there is a risk for that the vapor filled cavity collapses and molten metal penetrates into the backside of the work piece. The use of backings, that is a well known method to prevent that molten material flows out from the gap between the edges and may be lost, may then become necessary. In some cases though, the use of backings may be physically prevented due to limited access to the back side of the work piece.
Another problem that may arise when forming weld seams between thick work pieces is that the weld seam may become deteriorated due to formation of hard brittle zones around the joint. The hard brittle zones are formed during rapid cooling at the seam. It has been shown that when increasing the thickness of the work pieces to be joined, the problem with formation of localized hard brittle zones may increase. The problem of formation of localized hard brittle zones is also increased when welding high strength steels, that is steels having a yield strength exceeding 355 MPa. When using such materials, the peaks of material hardness in the zones surrounding the joint become even higher than in comparative welds in less strong materials.